1. Field of the Invention
The present invention relates to a sheet member holding apparatus wherein a pair of arms, which extend in opposite peripheral directions from a fulcrum attached to a part of a peripheral surface of a rotary body, are pivoted about the fulcrum by a centrifugal force generated by rotation of the rotary body, one of the arms approaches the peripheral surface of the rotary body, and a sheet member wound around the peripheral surface of the rotary body is held by a clamp section attached to the one of the arms.
2. Description of the Related Art
For printing, a photosensitive printing plate (hereinafter, xe2x80x9cprinting platexe2x80x9d), in which a photosensitive layer is formed on a sheet-type supporting body such as a thin plate made of aluminum, is generally used. As such printing plates, printing plates having different longitudinal and breadthwise dimensions according to printing size are used.
An image exposure apparatus for performing image exposure on a printing plate includes an apparatus which winds a printing plate around a rotary drum and then emits a light beam based on image data to the printing plate while rotating the printing plate integrally with the rotary drum, so as to scan-expose the printing plate.
When the printing plate is wound around the rotary drum, the printing plate is nipped and fixed to the rotary drum by holding apparatuses (chucks) for clamping both ends, in the peripheral direction of the rotary drum, of the printing plate (nipping the plate between the chucks and the peripheral surface of the rotary drum).
That is, a holding apparatus corresponding to one end of the printing plate along the drum peripheral direction (for example, the end at a winding front end side) is attached to a predetermined position of the rotary drum, and, after the printing plate has been wound around the rotary drum, a holding apparatus corresponding to the other end of the printing plate (here, the end at a winding rear end) is attached to a position according to the size of the printing plate.
Here, the holding apparatus for attaching the printing plate to the rotary drum at the position according to the size has a fixing barrel (stanchion) which can be inserted into an arbitrary position of an attachment groove which is formed along the peripheral direction in the peripheral surface of the rotary drum. The fixing barrel attached to the holding apparatus is freely movable relative to the attachment groove and can nip the ends of printing plates having various sizes in suitable positions.
When the printing plate wound around the rotary drum is to be exposed, the printing plate is closely contacted with the peripheral surface of the rotary drum and rotated at high speed together with the rotary drum.
At this time, in order to prevent lifting of the end of the printing plate fixed by the clamp section of the holding apparatus due to centrifugal force, a structure in which the centrifugal force is utilized to instead increase holding force of the clamp section has been suggested (for example, see Japanese Patent Application Laid-Open No. 2000-112142).
In this publication (of the prior art), because force is applied by the centrifugal force to portions to be held, a ratio of lengths from a fulcrum to the ends of a seesaw-type arm (a mechanism where a center portion is the fulcrum and the arm pivots about the fulcrum) is prescribed, and a strong holding force is applied at a distal end of the arm at a clamp section side thereof. In this prior art, a combined structure in which the sheet material is adsorbed to the rotary drum when wound around the rotary drum is also used.
However, in the above prior art, holding of the sheet material is mainly due to the adsorption, and holding reinforcement due to the centrifugal force is utilized supplementarily. Moreover, the publication discloses only prescription of the lengths of the arms and simply describes the well-known law of moment. Therefore, a position of the center of gravity in the holding apparatus, a coefficient of friction between the holding apparatus and the sheet material at the time of holding the sheet material, a coefficient of friction between the sheet material and the rotary drum, and the like are not taken into consideration.
However, in a case where the sheet material is not adsorbed to the rotary drum, the above coefficients of friction and the like can be important conditions, and the centrifugal force cannot be utilized effectively merely by prescribing the lengths of the arms.
It is an object of the present invention to provide an apparatus which utilizes centrifugal force effectively so as to be capable of holding a sheet member securely.
There will now be explained a first principle with reference to a model shown in FIGS. 1A and 1B.
FIG. 1A shows a state in which a sheet member 202 is wound around a peripheral surface of a rotary body 200. A radius of the rotary body 200 is r, a thicknesswise dimension of the sheet member 202 is t, and density of material of the sheet member 202 is c.
A holding apparatus 204 of the present invention is provided at an end of the sheet member 202, and a pair of arms 208 and 210 are provided extending from a fulcrum 206 in opposite directions along a peripheral direction of the rotary body 200. A first of the arms 208 (left of the fulcrum 206 in FIG. 1A) is pivoted on the fulcrum 206 by centrifugal force generated at a time of rotation of the rotary body 200 so as to approach the peripheral surface of the rotary drum 200, and the sheet member 202 is consequently pressed by a clamp section 212. As a result, the sheet member 202 is nipped and held between the peripheral surface of the rotary body 200 and the clamp section 212.
There is a coefficient of friction xcexc1 between the clamp section 212 and the sheet member 202, and a coefficient of friction xcexc2 between the sheet member 202 and the rotary body 200.
FIG. 1B is an enlarged diagram of the holding apparatus 204 of the present invention.
Length of the first arm 208 (from the fulcrum 206 to the clamp section 212) is L1, length of the second arm 210 (from the fulcrum 206 to a center of gravity 214) is L2, and a unit weight of the center of gravity (weight per unit length of the arm in an axial direction of the rotary body) is m.
Here, when the rotary body 200 rotates, a centrifugal force mxc3x97rxc3x97xcfx892 is applied at the center of gravity 214 in an upward direction of FIG. 1B. Here, xcfx89 is angular velocity.
This centrifugal force effects a holding force at the clamp section 212 of the first arm 208 to the sheet member 202, and when a ratio of the lengths of the first arm 208 and the second arm 210 is taken into consideration, theoretically, the sheet member 202 is nipped by a force of (L2/L1)xc3x97mxc3x97rxc3x97xcfx892.
Further, because the coefficients of friction xcexc1 and xcexc2 are effective at the clamp section 212, when the coefficients of friction xcexc1 and xcexc2 are combined, the holding force of the clamp section 212 generated by the rotation of the rotary body 200 can be represented by the following expression.
(xcexc1+xcexc2)xc3x97(L2/L1)xc3x97mxc3x97rxc3x97xcfx892 
On the other hand, as a force which acts at the sheet member 202 during the rotation of the rotary body 200, a force due to the centrifugal force and acting to pull the sheet member out of the clamp section 212 may be considered. This force can be represented by the density c and thicknesswise dimension t of the sheet member 202, and the radius r and angular velocity xcfx89 of the rotary body 200 in the following expression.
cxc3x97r2xc3x97txc3x97xcfx892 
In order to hold the sheet member 202 securely by the clamp section 212, the following relationship must hold:
(xcexc1+xcexc2)xc3x97(L2/L1)xc3x97mxc3x97rxc3x97xcfx892 greater than cxc3x97r2xc3x97txc3x97xcfx892 
When common denominators (r and xcfx892) are removed, the following relationship holds.
{(xcexc1+xcexc2)xc3x97(L2/L1)xc3x97m} greater than cxc3x97rxc3x97t 
Conversely, the coefficients of friction xcexc1 and xcexc2 the length L1 of the first arm 208 (from the fulcrum 206 to the clamp section 212), the length L2 of the second arm 210 (from the fulcrum 206 to the center of weight 214), the unit weight m of the center of gravity 214 (weight of the arm per unit length in the axial direction of the rotary body), the density c and thicknesswise dimension t of the sheet member 202, and the radius r of the rotary body 200 can be set suitably such that an ideal holding apparatus for utilizing the centrifugal force can be structured.
Next, there will now be explained a second principle of the present invention with reference to a model shown in FIGS. 2A and 2B. The same reference numerals are given to elements which are substantially the same as those in FIGS. 1A and 1B, and overlapping explanations are suitably omitted.
FIG. 2A shows a state in which the sheet member 202 is wound around the peripheral surface of the rotary body 200. The radius of the rotary body 200 is r, the thicknesswise dimension t of the sheet member 202 is t, and the density of material of the sheet member 202 is c.
The holding apparatus 204 of the present invention is provided at an end of the sheet member 202, and the pair of arms 208 and 210 are provided extending from the fulcrum 206 in opposite directions of the peripheral direction of the rotary body 200. The first arm 208 (left of the fulcrum 206 in FIG. 2A) is pivoted about the fulcrum 206 by the centrifugal force generated at the time of the rotation of the rotary body 200 so as to approach the peripheral surface of the rotary drum 200, and the sheet member 202 is pressed by the clamp section 212. As a result, the sheet member 202 is nipped to be held by the peripheral surface of the rotary body 200 and the clamp section 212.
There is a coefficient of maximum rest friction xcexc1 between the clamp section 212 and the sheet member 202, and a coefficient of maximum rest friction xcexc2 between the sheet member 202 and the rotary body 200.
FIG. 2B is an enlarged diagram of the holding apparatus 204 of the present invention.
The length of the first arm 208 (from the fulcrum 206 to the clamp section 212) is L1 the length of the second arm 210 (from the fulcrum 206 to a center of gravity 214) is L2, and the unit weight of the center of gravity 214 (weight of the arm per unit length in an axial direction of the rotary body) is m. A distance from the center of the rotary body 200 to the center of gravity 214 is xcex1r.
Here, when the rotary body 200 rotates, a centrifugal force mxc3x97xcex1xc3x97rxc3x97xcfx892 is applied at the center of gravity 214 in an upward direction of FIG. 2B.
The centrifugal force effects the holding force at the clamp section 212 of the first arm 208 to the sheet member 202, and when the ratio of the lengths of the first arm 208 and the second arm 210 is taken into consideration, theoretically, the sheet member 202 is nipped by a force of (L2/L1)xc3x97mxc3x97xcex1xc3x97rxc3x97xcfx892. Moreover, an additional pressure force T0 may be applied as a nipping force in a ratio L3/L1, where L3 is a distance from the fulcrum 206 to a point of application of the pressure force T0. Thus, a value obtained by adding the centrifugal force and the pressure force becomes an overall holding force ((L2/L1)xc3x97mxc3x97xcex1xc3x97rxc3x97xcfx892+(L3/L1)).
Here, if the sheet member 202 is drawn in a stretching direction by the clamp section 212, a condition such that, due to friction between the clamp section 212 and the sheet member 202, sliding does not occur is required. Therefore, if a coefficient of maximum rest friction is xcexc1, and a tensioning force from the clamp section 212 generated by the rotation of the rotary body 200 is F, the following expression should hold.
xe2x80x83xcexc1[(L2/L1)mxcex1rxcfx892+(L3/L1)T0] greater than F 
A similar holding force is applied between the sheet member 202 and the rotary body 200. At this time, the tensioning force F pulling the sheet member should be larger than a value obtained by multiplying this holding force by the friction coefficient xcexc2 between the sheet member 202 and the rotary body 200. Namely, the following expression should hold.
F greater than cr2txcfx892+xcexc2[(L2/L1)mxcex1rxcfx892+(L3/L1)T0]
The above two expressions are combined as follows.
xcexc1[(L2/L1)mxcex1rxcfx892+(L3/L1)T0] greater than F greater than cr2txcfx892+xcexc2[(L2/L1)mxcex1rxcfx892+(L3/L1)T0]
The coefficients of maximum rest friction xcexc1 and xcexc2, the length L1 of the first arm 208 (from the fulcrum 206 to the clamp section 212), the length L2 of the second arm 210 (from the fulcrum 206 to the center of gravity 214), the unit weight m of the center of gravity 214 (weight of the arm per unit length in the axial direction of the rotary body), the distance from the center of the rotary body to the center of gravity xcex1r, the pressure force T0 applied to the sheet member 202 in the pressing direction by the clamp section 212 when the rotary body 200 is still, the distance L3 from the fulcrum to the position to which the pressure force T0 is applied, the density c and the thicknesswise dimension t of the sheet member 200, and the radius r of the rotary body 200 can be set such that the above relational expression is satisfied. As a result, the sheet member can be fixed securely. Namely, the sheet member can be put into a stretched state (lift prevention state) by an ideal tensioning force utilizing the centrifugal force.
In the above relational expression, the tensioning force F is a force stretching the sheet member in a stretching direction due to the centrifugal force. If the tensioning force F is krxcfx892 (k is a coefficient relating to mass) and the pressure force T0 is 0, the following expression can be obtained:
xcexc1(L2/L1)mxcex1 greater than k greater than crt+xcexc2(L2/L1)mxcex1
Position, weight and the like of the members in the assembled holding apparatus, rotary body and the like relating to the coefficient k can be set such that the above expression is satisfied. The pressure force T0 may be a weak force such that the sheet member is not allowed to move by the holding apparatus. Accordingly, a degree of freedom of design can be increased.